The best source I found is Edward Hallowell's book Dare to Forgive. My ex N (we weren't married) asked for instant forgiveness after pulling off a big betrayal which he lied about at first. I searched the net about forgiveness and then bought the book.
Remember that forgiveness is something you do for yourself, not for the other person. He/she may never know that you've achieved forgiveness (working the nastiness out of your system so it no longer affects/hooks you). It doesn't mean that you condone the behavior, or want to see the person again, you just finally accept that it happened, probably learn from it, recognize finally the person's limitations, and move on. Achieving forgiveness is proven good for your health too. Depending on how deep the hurt is, it may take time.